Recognition that the macroscopic properties of materials depend not only on their chemical composition, but also on the size, shape and structure, has spawned investigations into the control of these parameters for various materials. In this regard, the fabrication of uniform hollow spheres has recently gained much interest. Hollow capsules with nanometer and micrometer dimensions offer a diverse range of potential applications, including: utilization as encapsulants for the controlled release of a variety of substances, such as drugs, dyes, proteins, and cosmetics. When used as fillers for coatings, composites, insulating materials or pigments, hollow spheres provide advantages over the traditional solid particles because of their associated low densities. Hollow spheres may also be used in applications as diverse as hierarchical filtration membranes and proppants to prop open fractures in subterranean formations. A spherical morphology also allows for applications in optical devices.
The geometry of the spheres has been shown to increase the strength of composite materials. Incorporating hollow spheres into composite materials improves the strength and the fracture strength of the material. Typically, materials (organic or inorganic) are reinforced with fibers that retard the propagation of stress cracks. When hollow particles are incorporated into the fiber-reinforced composite, the crack growth is further stopped by the neighboring particles, for example, incorporation of glass beads into an epoxy resin.
Hollow particles have been fabricated from a variety of materials, such as polymers, metal, ceramics, and glass, however, a great deal of research has focused on various metal oxides, due to their chemical, thermal, and oxidative resistance, and because they have low dielectric constants and are optically transparent. Conventional methods to produce hollow ceramic spheres are vapor deposition, sputtering, molecular beam deposition and electrolytic deposition; however, these processes do not always provide a uniform coating of individual particles. Ceramic spheres exhibiting a uniform coating and thickness have been achieved with the sol-gel route. Typically the spheres are formed by templating with either polystyrene spheres or silica spheres. The polystyrene or silica spheres are coated with the sol-gel, after which the core is etched away, and calicination results in a ceramic hollow sphere. Titanium dioxide, barium titanate, alumina, and aluminosilicate spheres have been fabricated using the sol-gel templating technique.
It has previously been shown that for alumina films and bodies, a low cost, flexible, alternative to sol-gels are chemically functionalized alumina nanoparticles (carboxylate-alumoxanes) (R. L. Callender, C. J. Harlan, N. M. Shapiro, C. D. Jones, D. L. Callahan, M. R. Wiesner, R. Cook, and A. R. Barron, Chem. Mater. 9 (1997) 2418, incorporated herein by reference). These alumina nanoparticles may be prepared, in the size range 10-100 nm with a narrow size distribution, by the reaction of the mineral boehmite with a wide range of carboxylic acids. Besides the use of aqueous reaction conditions, without mineral acids or other additives (resulting in high ceramic yields and low shrinkage), the carboxylate-alumoxanes are stable in solution or the solid state (i.e., they do not precipitate or undergo changes in particle size ordinarily associated with aging of sol-gels). Carboxylate-alumoxanes may be used as ceramic precursors for the coating on carbon, SiC and Kevlar fibers (as demonstrated in R. L. Callender and A. R. Barron, J. Mater. Res. 15 (2000) 2228, herein incorporated by reference).
A further advantage of the carboxylate-alumoxane nanoparticle approach is that the porosity of the ceramic formed upon thermolysis may be controlled by the substituent of the carboxylic acid, which has led to their application as precursors for ceramic membranes. A final advantage of the alumoxane approach over traditional sol-gel is the ease by which aluminate phases may be prepared, often at a lower temperature than previously observed. Thus, carboxylate-alumoxanes may be used to create hollow spheres of alumina or an aluminate. Furthermore, it has previously been shown that layer-by-layer (LbL) growth of laminates is possible, which opens-up the possible fabrication of ceramic composites with increased applications, such as the formation of magnetic materials (Z. Y. Zhong, T. Prozorov, I. Felner, and A. Gedanken, J. Phys. Chem. 103 (1999) 947, herein incorporated by reference).
Alumina sol-gel-derived membranes are presently the most accepted routes to making alumina ultrafiltration filters. Lennears, Keizer, and Burgraff first developed the technique of using sol-gel processes to make alumina ultrafiltration membranes. These filters, along with the vast majority of those reported in the literature, were made by the controlled hydrolysis of aluminum alkoxides to form alumina. The preparation techniques used by various researchers vary the drying or sintering conditions, which result in small changes in porosity or pore size. The membrane selectivity is primarily dependent upon the pore-size distribution; the narrower the pore size distribution, the more selective the membrane. However, for membranes produced by sol-gel techniques the pore size is generally limited to the size distribution of the precursor particles before sintering, which is difficult to control. Furthermore, sol-gels must be used immediately after preparation to avoid aggregation or precipitation.
It has previously been reported that the fabrication of asymmetric alumina ultrafiltration membranes may be accomplished using carboxylic acid surface stabilized alumina nanoparticles (carboxylate-alumoxanes) (D. A. Bailey, C. D. Jones, A. R. Barron, and M. R. Wiesner, “Characterization of alumoxane-derived ceramic membranes”, J. Membrane Sci., 176, (2000), 1-9 and C. D. Jones, M. Fidalgo, M. R. Wiesner, and A. R. Barron, “Alumina ultrafiltration membranes derived from carboxylate-alumoxane nanoparticles”, J. Membrane Sci., 193, (2001), 175-184, herein incorporated by reference). A comparison with membranes derived from sol-gel methods showed carboxylatealumoxane-based membrane properties to be favorable. The synthesis of the alumina nanoparticles is simple and low cost, producing a defect free membrane in a one-step process. For example, the carboxylate-alumoxane nanoparticles may be prepared in sufficient quantity for 100-200 m2 of finished membrane in a single laboratory-scale batch, with the cost of raw material being less that $5. Once prepared, the carboxylate-alumoxanes are stable for months in solution, or may be dried and redissolved as desired, without changes in particle size.
For both sol-gel and the present carboxylate-alumoxane derived membranes, mechanical integrity and permeability may be enhanced by supporting a relatively thin selective membrane on a thicker, more permeable substrate so as to produce an asymmetric membrane. Despite this approach, and due to the small pore size of the alumoxane-derived membranes, the permeability of the asymmetric membranes is significantly lower than that of the support. In order to approach the permeability of the support, the ultrafiltration membrane must be as thin as possible. Sol-gel derived membranes often require multiple dip-fire sequences to ensure integrity. In contrast, a single step process is sufficient for the alumoxane nanoparticle approach. Unfortunately, in order to ensure a defect free membrane, a thickness of 1-2 μm is required. Thus, an alternative approach must be used in both processes in order to increase flow.
A typical technique for constructing a sol-gel membrane is to layer materials of different porosity such that the thinnest possible layer of the “effective” ultrafiltration membrane is provided. However, this also required multiple process steps and each layer may result in a decrease in flux. If decreasing membrane thickness is not practical, an alternative approach is to increase the macroscopic surface area of the membrane. By analogy with biological membranes, one proposal is the creation of a hierarchical structure, in which the macroscopic structure evolves through ever decreasing sizes. A good example of such a structure would be the mammalian lung. A hierarchical approach has previously been used for organic membranes and mesoporous materials (see T. Kawasaki, M. Tokuhiro, N. Kimizuka, T. Kunitake, “Hierarchical self-assembly of chiral complementary hydrogen-bond networks in water”, J. Am. Chem. Soc., 123 (2001) 6792-6800 and H.-P. Lin, Y.-R. Cheng, C.-Y. Mou, “Hierarchical order in hollow spheres of mesoporous silicates”, Chem. Mater. 10 (1998) 3772-377, incorporated herein by reference).